Ion source having a hollow cylindrical permanent magnet maintained at a positive potential relative to the electron emitter



y 28, 1963 I w. D. DAVIS 3,385,965

IoN SOURCE HAVING A HOLLOW CYLINDRICAL PERMANENT MAGNET MAINTAINED AT APOSITIVE POTENTIAL RELATIVE To THE ELECTRON EMITTER Filed Aug. 10, 1965His Afro/nay.

High Voltage 0. 6. Power PP y United States Patent [0N SOURCE HAVING AHOLLOW CYLINDRKCAL PERMANENT MAGNET MAINTAINED AT A POSITliVE PDTENTHALRELATK" E TO THE ELEC- TRON EMITTER William D. Davis, Schenectady, N.Y.,assignor to General Electric Company, a corporation of New Yorir FiledAug. 1t), 1965, Ser. No. 478,668 5 Claims. (Cl. ISO-41.9)

ABSTRACT (9F THE DISCLGSURE A mass spectrometer ion source for highlyefficient production of ions at very low pressures having substantiallyno energy spread utilizes a magnetic field to enhance ionization.Electrons emitted from a hairpin filament situated within the interiorof a hollow cylindrical permanent magnet are constrained by electricfields to remain within the interior of the magnet and thereby increasethe probability of collisions with gas molecules.

This invention relates to mass spectrometer ion sources and moreparticularly to an improved electron bombardment type ion source whichis especially efiicient at very low gas pressures. The invention hereindescribed was made in the course of or under a contract or subcontractthereunder with the Air Force Department.

Gas analysis by mass spectrography requires production of sufficientnumbers of ions to achieve reasonably high output signals. Thisrequirement becomes more difficult to meet as gas pressures aredecreased. In electron bombardment type ion sources operating at verylow pressures and producing electrons thermionically, it is necessary tokeep electron emission and filament heater power low in order to avoidoutgassing; however, enough ions must still be produced in order toenable measurement of the partial pressures. At these very lowpressures, therefore, the ratio of ion current to electron emission mustbe made as high as possible.

Heretofore, mass spectrometers utilizing conventional ion sources havebeen unsuited for applications at extremely low pressures, since theattendant outgassing necessitated much pumping of the ion source.Moreover, mass spectrometer ion sources utilizing magnetic fields forenhancement ionization exhibit diminished resolution of the spectrometerto below satisfactory values because of a high energy spread among theions. Moreover, because use of mass spectrometers at very low pressuresrequires that the spectrometer first be baked at temperatures of about400 C. in order to remove extraneous particles, materials used withinthe spectrometer, such as organic insulation for wire, as well as thematerial of the spectrometer itself, tend to emit contaminant particlesduring the baking process. Thus in mass spectrometers utilizingconventional ion sources, there eXists a level of gas pressure belowwhich it is impractical to attempt to make an accurate gas analysis.

Accordingly, one object of this invention is to provide a massspectrometer ion source for permitting accurate gas analyses to be madeat pressures lower than have heretofore been possible.

3,385,955 Patented May 28, 1968 Another object is to provide anefficient source of ions having substantially no energy spread.

Another object is to provide a mass spectrometer ion source using apermanent magnet as the anode thereof.

Another object is to provide a mass spectrometer ion source having aminimal possibility of contamination from the materials of which the ionsource is constructed.

Briefly stated, in accordance with one aspect of the invention, a massspectrometer ion source is provided having a first end plate containingan aperture therein and a hollow cylindrical permanent magnet having oneend concentrically aligned with the aperture. At the other end of thepermanent magnet, a second end plate is situated in orthogonal relationto the longitudinal axis of th magnet. Electron emitting means aresituated within the hollow interior of the magnet and are maintained ata potential which is positive with respect to the end plates andnegative with respect to the magnet.

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in connection with the accompanyingdrawing in which:

The single figure is a cross section view of the portion of a massspectrometer tube containing the novel ion source, with some of thedetail illustrated in cutaway and some illustrated schematically.

In the drawing, the ion source assembly is shown mounted within anevacuable envelope 10. One mass spectrometer tube adaptable for use withthe novel ion source described herein, is similar to that shown in theapplication of W. D. Davis et al., Ser. No. 327,617, filed Dec. 3, 1963,now Patent No. 3,230,362 issued Jan. 18, 1966, and hence only theportion of the mass spectrometer tube containing the ion source is shownherein. The ion source basically comprises a thermionic electronemitting filament 11 having its apex portion 12 protruding into thehollow interior 13 of a cylindrically-shaped permanent magnet 14 throughan aperture 15 in a metallic end plate 16 situated at one end of thecylindrical permanent magnet. The permanent magnet is preferablyconstructed of Alnico-8, although Alnico-S is also a satisfactory magnetmaterial. A second metallic plate 17 having an aperture 18 therein issituated at the opposite end of the cylindrical permanent magnet. Thelongitudinal axis of cylindrical permanent magnet 14 is concentricallyaligned with apertures 15 and 18 of plates 1t; and 17 respectively, andis situated in orthogonal relation to the plates. Apertures 15 and 18are preferably circular in configuration.

Drawout and focus electrodes 19 and 20 are situated above end plate 17.Each of these electrodes comprises a pair of opposed, substantiallysemicircular discs located in a common plane. The planes of electrodes19 and 20 are parallel to the plane of end plate i7. Each half of lowerfocus electrodes 19 has an upward-turned flange 21 along the entirechordal or diametral portion of the electrodes; similarly, each half ofupper focus electrode 20 has a downward-turned flange 22 along theentire chordal or diametral portion of the electrode. Focus electrodes19 and 20 are situated so that the transverse spacing between flanges ofthe respective portions of each electrode is bisected by thelongitudinal axis of cylindrical permanent magnet 14.

Situated above the focus electrodes and in parallel relationship theretoare a first metallic disc 23 and a second metallic disc 24. Each dischas a coliirnating slit therein, with slit 25 in disc 23 being severaltimes wider than slit 26 in disc 24 in order to narrow the beam of ionspassing therethrough. The lengthwise portions of these slits areoriented parallel to the chordal or diametral portions of electrodes 19and 29, and their widths are bisected by the longitudinal axis ofcylindrical permanent magnet 14. Although the functions of the focus andcollimating electrodes are well-known in the art, means for applying therequisite voltages thereto are described herein.

The ion source is supported by means of four support rods 27, only twoof which are shown. These support rods are fabricated of dielectricmaterial, such as quartz. A collar 28 forming an integral portion ofeach support rod abuts collimating disc 24, which is Welded or brazed toa metallic plate 29. An extension portion 30 of each support rod 27passes through mounting holes in both collimating disc 24 and metallicplate 29. Metallic plate 29 comprises a flange which is welded or brazedto a sector tube 31, which passes through a metallic cap 32 to form thecharacteristic deflection portion of the mass spectrometer. Cap 32 isfitted to envelope 10 in order to form a hermetic seal therewith, andsector tube 31 is welded or brazed to cap 32 for the same purpose. Atubulation is provided for the evacuable assembly, as for example,tubulation 33 of evacuable envelope 10, for connection to suitablepumping apparatus in order to obtain the required low operating pressuretherefor.

A plurality of spacers in the form of collars fabricated of a dielectricmaterial, such as quartz, are fitted around each of support rods 27 forelectrically isolating the various electrode structures. Thus, spacers34 separate collimating electrode 23 from drawout and focus electrode20, spacers 35 separate drawout end focus electrodes 19 and 20 from eachother, spacers 36 separate drawout and focus electrode 19 from end plate17, spacers 37 separate end plate 17 from a first anode electrode 43,spacers 38 separate first anode electrode 43 from a second anodeelectrode 44, spacers 39 separate second anode electrode 44 from an endplate support ring 45, and spacers separate end plate support ring 45from a non-conducting ceramic ring 46. The ceramic ring is retained inposition by coil springs 47 which are tightly wound around each ofsupport rods 27, respectively.

Four holes 48, only two of which are shown, are bored through ceramicring 46, and a shaft 59 is respectively inserted therein. Each shaft 50is joined to a mounting pin 51, respectively. Alteratively, shaft 56 maybe constructed as an integral extension of mounting pin 51. At eachjunction of shaft 5t with mounting pin 51 there is a flanged collar 53,affixed by welding or other means, to mounting pin 51. An elongated coilspring 54 is wound in compression, around each shaft 50, respectively,thus exerting an upward force against ceramic ring 46, tending tocompress spacers 34-40. Because of the force exerted by each of springs54, the ion source assembly is retained as an integral unit. Inaddition, springs 54 also function as shock absorbers in the event anyof mounting pins 51 are moved vertically due to temperature changes orother reasons.

Support for cylindrical permanent magnet 14 is provided by a pair oftabs 55 which are affixed to opposite sides of the outer surface of thepermanent magnet by spot welding or other means. First and second anodeelectrodes 43 and 44 respectively are preferably welded to the upper andlower portions of tabs 55. Anode potential may then be supplied to thepermanent magnet through a conductor wire 56 which is attached to amounting pin 57 by welding or other-wire. A flexible loop of wire, orpigtail 58 may be inserted in the conducting path of wire 56, in orderto permit slight movement of the ion source, when necessary, withoutcausing damage to the source. in similar fashion, a potential issupplied to end plates 16 and 17 through a conductor 60 having a pigtail61 inserted therein, through a mounting pin 62. Because positivepotential exists on permanent magnet 14 with respect to potentialapplied to end plates 16 and 17, end plate support ring 45 must have asufliciently large aperture therein in order to avoid contact with thecylindrical permanent magnet. Therefore, less positive voltage for endp. e i6 is supplied through a conductor 63 which is welded between endplate support ring 45 and end plate 16. A pair of nonconducting ceramicdiscs 64 are affixed to end plate support ring 45 such as by a bolt 74,and each disc 64 is used to provide support through a bolt '75 or othermeans, for a respective conductor 65. A conductor 6-6 is welded betweenone end of each of conductors 65 and separate sides of filament 11,respectively. Welded between each of a pair of filament support pins 67and each of the other ends of conductors 65, respectively, is aconductor 68, respectively. Each of conductors 63 contains a pigtail 70therein.

Each portion of drawout and focus electrode 19 is connected through aconductor 76 containing a pigtail 77 therein to a mounting pin 78, andare thereby maintained at a common potential. Each portion of drawoutand focus electrode 20 is separately connected through a conductor 80containing a pigtail 82 and a conductor 81 containing a pigtail 83respectively, to mounting pins 84 and 85 respectively. Collimatingelectrodes 23 and 24 are connected through a conductor 86 containing apigtail 87 and a mounting pin 88 to ground.

D.C. filament voltage is supplied to support pins 67 from a D.C. powersupply or storage battery 71, although A.C. filament voltage mayalternatively be supplied. A high voltage D.C. power supply 72 providesa large positive potential through mounting pin 57 to cylindrical magnet14, and a lesser positive voltage to end plates 16 and 17 through abucking battery 73 connected to mounting pin 62. By connecting a battery90 between filament D.C. power supply 71 and high voltage D.C. powersupply 72, of voltage less than that of battery 73 and poled so as tobuck the high voltage supply, filament 11 is maintained at a positivepotential intermediate the mode and end plate potentials.

Positive voltage is supplied to both halves of focus electrode 19 fromhigh voltage power supply 72 through a variable resistor 91 connected tomounting pin 78. Each half of focus electrode 20 is maintained at aseparate positive voltage supplied from high voltage D.C. power supply72 through respective parallel-connected variable resistors 92 and 93connected to mounting pins 84 and 85 respectively. Hence, voltagemagnitudes and polarities may be established independently on electrodes19 and 20. It should be noted that the mounting pins penetrate envelope10 in a manner which maintains the hermetic seal of the envelope.

The composition of the ion source permits removal of extraneousparticles by baking the system at high temperatures, without influencingthe amount and composition of the gas to be analyzed. These extraneousparticles have proven to be a source of inaccuracy, par ticularly inmass spectrometer tubes requiring an electromagnet within the ion sourceassembly, due to spurious emission of particles from the insulationrequired for the windings of the electromagnet. Elimination of theelectromagnet, therefore, substantially obviates this problem.

Because the apex portion of the filament extends into the hollowinterior of magnet 14, which produces a relatively weak magnetic field,lower anode voltages than heretofore deemed practical may be used. Sinceelectrons emitted from the filament thus have very little lateraldistance to travel before striking the anode, the anodic field must notbe of sufficient magnitude to immediately draw electrons to the innersurface of the anode. This condition is especially important atextremely low pressures, when it is desirable that electrons travel overgreat distances within the interior of the cylindrical permanent magnetin order to enhance the possibility of colliding with a gas molecule andproducing an ion. Due to presence of end plates 16 and 17, the electronsemitted within the interior of permanent magnet 14 are constrained toremain therein, increasing the probability of obtaining a longer meanelectron path within the interior of the magnet.

Utilization of a permanent magnet as the anode results in a moreconfined magnetic field than would be obtainable from an electromagnetsurrounding the anode and producing the same flux density. This isadvantageous in that the magnetic field thus has less disturbing effectupon the portion of the mass spectrometer tube outside the ion source.Moreover, because the magnetic field is so confined, proper ope-rationof the ion source requires less total magnetic flux from the permanentmagnet anode than from an electromagnet surrounding the anode. Thislesser value of required flux is an amount which is readily obtainedfrom a permanent magnet.

In typical operation, the anode is maintained at approximately +1000volts by high voltage power supply 72, while the end plates aremaintained at approximately +750 volts due to a bucking potential of 250volts supplied by battery 73 in series with power supply 72. Thefilament may be operated at 1 or 2 volts DC. from power supply 71, whilebeing maintained at approximately +950 volts above ground potential byvirtue of a 50 volt potential introduced by battery 90 in seriesopposition with power supply 72. Electrons emitted by the hot filamentare repelled by the end plates which are thus 200 volts negative withrespect to the filament, and attracted to the anode which is 50 voltspositive with respect to the filament. The axial magnetic field producedby permanent magnet 14, which may be in the order of about 500 gauss,constrains the movement of electrons to a reciprocating motion withinthe cylinder, thereby making the path of the electrons much greater thanthe dimensions of the cylinder. Although the exact behavior of theelectron cloud within the cylinder is uncertain, the net result is alarge increase in the ionization efficiency of the electrons. The ionsproduced within the interior of cylindrical permanent magnet 14 arewithdrawn through aperture 18 in end plate 17, and are focussed andaccelerated in conventional fashion, such as described in thestore-mentioned Davis et al. Patent No. 3,230,362.

It is noted that conventional electron bombardment type ion sources formass spectrometers have typical sensitivities for generating ions ofabout torr* senitivity being measured by the ratio of ion current to theproduct of electron emission and pressure. On the other hand, by usingthe novel ion source described herein, a source sensitivity of 1.2 10torrhas been attained at a pressure of 3x10 torr and electron emissioncurrent of 10" amperes. Similar results have been obtained at a pressureof 3X10 torr. This drastic increase in sensitivity permits generation ofless electrons, thus allowing use of low heater power in order to avoidoutgassing without reducing the number of ions generated to a value lessthan that required to accurately read the partial pressures. Moreover,use of low heater power greatly extends filament life. Further, underthe aforementioned conditions, resolution of the mass spectrometer hasbeen normal, indicating that the ions are supplied to the spectrometerwith no energy spread. In addition to the aforementioned advantages, thenovel ion source disclosed herein greatly increases the ratio ofordinary gas ions to those ions such as F Cl+, Na etc., which appear tobe formed by electrons striking the surfaces of the ion source. At verylow pressure, such surface ions could interfere with measurements of thegas ions,

in the event the number of ordinary gas ions were low.

While only certain preferred features of the mass spec trometer ionsource of the present invention have been shown by way of illustration,many modifications and changes will occur to those skilled in the art.It is, there fore, to be understood that the appended claims areintended to cover all such modifications and changes which fall with thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a mass spectrometer tube, an ion source assembly comprising: apair of end plates, each said end plate containing an aperture therein;a hollow cylindrical permanent magnet; support means mounted within saidtube for maintaining said permanent magnet longitudinally disposedbetween said end plates and concentrically aligned with said apertures;an electron emitter afiixed to said support means so as to protrudethrough one of said apertures into the hollow interior of said magnet;and circuit means coupled to said pair of end plates, said electronemitter, and said permanent magnet to maintain said pair of end platesat a negative potential with respect to said electron emitter and saidmagnet at a positive potential with respect to said electron emitter.

2. In a mass spectrometer tube, an ion source assembly comprising: apair of end plates, each said end plate containing an aperture therein;a hollow cylindrical permanent magnet; support means mounted within saidtube for maintaining said permanent magnet longitudinally disposedbetween said end plates and concentrically aligned with said apertures;a conducting filament adapted to be heated to incandescence by passageof current therethrough and afiixed to said support means so as toprotrude through one of said apertures into the hollow interior of saidpermanent magnet, said filament constituting a thermionic source ofelectrons; and circuit means coupled to said pair of end plates, saidfilament, and said permanent magnet to maintain said pair of end platesat a negative potential with respect to said filament and said magnet ata positive potential with respect to said filament.

3. In a mass spectrometer tube, an ion source assembly comprising: apair of end plates, each said end plate containing a circular aperturetherein; a hollow cylindrical permanent magnet; support means mountedwithin said tube for maintaining said permanent magnet longitudinallydisposed between said end plates and concentrically aligned with saidcircular apertures; a hairpin-shaped electron emitting filament affixedto said support means so that the apex portion thereof extends throughone of said circular apertures into the hollow interior of said magnet;circuit means coupled to said pair of end plates, said filament, andsaid permanent magnet to maintain said pair of end plates at a negativepotential with respect to said filament and said magnet at a positivepotential with respect to said filament; and means coupled to saidfilament for heating said filament.

4. In a mass spectrometer tube, an ion source assembly comprising: afirst end plate containing an aperture therein; a hollow cylindricalpermanent magnet support means mounted within said tube for maintainingone end of said magnet concentrically aligned with said aperture; asecond end plate, said support means maintaining said second end platein orthogonal relation to the longitudinal axis of said magnet at theother end of said magnet; electron emitting means affixed to saidsupport means so as to be situated within the hollow interior of saidmagnet; and circuit means coupled to said end plates, said electronemitting means, and said permanent magnet to maintain each of said endplates at a negative potential with respect to said electron emittingmeans and said magnet at a positive potential with respect to saidelectron emitting means.

5. In a mass spectrometer tube, an ion source assembly comprising: apair of end plates, each said end plate con- 7 taining an aperturetherein; a hollow cylindrical permanent magnet; support means mountedwithin said tube for maintaining said magnet longitudinally disposedbetween said end plates, said end plates being afiixed to said supportmeans so as to be in substantially orthogonal relationship with thelongitudinal axis of said magnet; electron emitting means afiixed tosaid support means so as to extend into the hollow interior of saidmagnet through the aperture of one of said end plates; and circuit meanscoupled to said pair of end plates, said electron emitting means, andsaid permanent magnet to maintain said pair of end plates at a negativepotential with respect to said electron emitting means and said magnetat a positive potential with respect to said electron emitting means.

References Cited UNITED STATES PATENTS 2,831,996 4/1958 Martina 313632,911,531 11/1959 Rikard et a1. 25041.9 3,109,115 10/1963 Lafferty324-33 X FOREIGN PATENTS 665,094 1/1952 Great Britain.

WILLIAM F. LINDQUIST, Primary Examiner.

